Technical Field
Embodiments of the invention relate generally to power generation systems and, more particularly, to a system and method for improving the performance of a selective catalyst reduction system in a heat recovery steam generator.
Discussion of Art
Gas turbines have been widely used to provide electric power, usually as a standby for both peak power and reserve power requirements in the utility industry. Gas turbines are preferred because of their rapid starting capability and low capital cost. Conventional gas turbines, however, operate with reduced thermal efficiency due to the high exit temperatures of the exhaust gas stream and the resulting thermal loss. Therefore, a gas turbine is often combined with a heat recovery steam generator to improve overall system efficiency.
As is known in the art, a heat recovery steam generator generates steam utilizing the energy in the exhaust from the gas turbine. In a cogeneration mode, steam produced from the heat recovery steam generator can be used for process applications, whereas in a combined-cycle mode, power may generated via a steam turbine generator.
All combustion processes utilizing fossil fuels have the potential for producing emissions such as nitrogen oxides (NOx) and carbon monoxide. This also applies to the combustion process in a gas turbine where there are high temperatures and high excess air levels. Accordingly, the exit gas from the gas turbine which flows into and through the heat recovery steam generator contains a significant quantity of NOx and carbon monoxide. Stringent environmental regulations for carbon monoxide and nitrogen oxides have led to the development of selective catalyst reduction (SCR) systems that have been integrated into heat recovery steam generators. SCR systems function to remove nitrogen oxides, for example, through a selective catalytic reduction process. Typically, ammonia is injected into the flue or exhaust gas passing through the heat recovery steam generator, which is then absorbed onto a catalyst, to convert the nitrogen oxides into nitrogen and water. The treated exhaust gas may then be exhausted to atmosphere.
In addition to controlling NOx emissions to within rather precise ranges, it is also desirable to minimize ammonia slip. Ammonia slip refers to emissions of unreacted ammonia that result from incomplete reaction of the NOx and the reagent. Ammonia slip can cause the formation of ammonium sulfates, which can plug or corrode downstream components, and can result in ammonia being absorbed into fly ash, which can affect disposal or reuse of the ash. Minimizing both ammonia slip and NOx emissions, however, are often competing objectives, which are made harder to meet by changing operating conditions, ageing of components, and other system variables.
In view of the above, there is a need for a system and method for improving the performance of a selective catalyst reduction system in a heat recovery steam generator. In particular, there is a need for a system and method of reducing NOx emissions while at the same time minimizing ammonia slip.